Euler-Euler and Euler-Lagrange Modeling of Wood Gasification in Fluidized Beds

Euler-Euler and Euler-Lagrange Modeling of

Wood Gasification in Fluidized Beds

Michael Oevermann Stephan Gerber Frank Behrendt

Berlin Institute of Technology

Faculty III: School of Process Sciences and Engineering Department of Energy Engineering

Chair for Energy Process Engineering and Conversion Technologies for Renewable Energies

Fasanenstr. 89, 10623 Berlin, Germany

Overview

Motivation

Modeling approaches

Euler-Lagrange / Discrete Element Method (DEM) Euler-Euler

Results

Motivation - simulation of wood gasification

Renewed interest in producing gas from biomass Gasification in fluidized bed reactors

Lack of detailed knowledge about the complex interactions between heterogeneous reactions and the fluid mechanics in fluidized beds

CFD modeling for particulate flow

Continuum models Euler-Lagrange DNS

tractable problem size

Euler-Lagrange model

Basis:

OpenFOAM (www.opencfd.co.uk/openfoam) Modules:

1 _{Solver for the gasphase (OpenFoam)}

2 _{Lagrangian particle tracking / DEM (OpenFOAM)} 3 _{Particle combustion / gasification model}

Euler-Lagrange model – gasphase

Mass / momentum balance:

(ρ)t + ∇ ·ρu = ˙ρs

(ρu)t + ∇ · {ρuu} + ∇p − ∇ · τ +ρg =Fs Energy / species balance:

(ρE )t+ ∇ · [(ρE + p)u] + ∇ · q =Q˙s

(ρuYα)t + ∇ · [ρYαu] −w˙α= ˙ρα,s

˙

Euler-Lagrange model – particle tracking

Equation of motion for every single particle mi d~vi dt + ~vi dmi dt = X j ~ Fij Ji d~ω dt + ~ω dJi dt = X j ~ Mij

Fluid dynamic forces ( drag, buoyant, Magnus, etc.) with empirical correlations

Contact forces via spring-damper modell Multiple particle contacts possible

Euler-Lagrange – contact forces by DEM

(Cundall & Strack, 1979)

i j kn

η

µ

η

Euler-Lagrange model – coupling

Detailed single particle

(Eulerian) Solid phase ˙ Q p, ˙ m p s Rp_{, ˙m} ρ g,~v g,~ω g,T g ~ F s, ˙ Q si,ε (e. g. Tp_{, Y}p s) φ (Lagrangian) iner t reacting ~rsi_{(t), ~v}si_(t) ~ ωpi_(t),Tsi_{(t), ˙Q}si ~rsr_{(t), ~v}sr_(t) ~ ωpr_(t) model (1d) Rp_{(t), m}p_{(t), T}p_(t) ˙ mps(t), ˙Qp(t) Particle library ˙ mp s(t; φ), ˙Qp(t; φ) Rp_{(t; φ), m}p_{(t), T}p_{(t; φ)} ρg_{(~x , t), ~v}g_{(~x , t)} Tg_{(~x , t), Y}g s(~x , t) Gas phase

Euler-Euler model

Public Domain Code MFIX (www.mfix.org) 1 gas phase

3 solid phases: wood (4mm), 2 x charcoal (1mm, 2mm) 5 homogeneous gas phase reactions (rev.),

4 heterogeneous reactions carbon

Gas phase chemistry

Simple reaction mechanism

Species: H2, O2, N2, CO, CO2, CH4, H2O, tar

1. 2 CO + O2 2 CO2

2. 2 H2 + O2 2 H2O

3. 2 CH4 + 3 O2 2 CO + 4 H2O

4. CO + H2O H2 + CO2

Pyrolysis / heterogeneous reactions

Primary pyrolysis (Grönli, Melan (2000))

wood → H2O(g), CO(g), CO2(g), H2(g), CH4(g), tar(g), char(s).

Secondary pyrolysis (Boroson et al. (1989))

tar → CO(g), CO2(g), H2(g), CH4(g), tarinert.

Heterogeneous reactions

C + O2 → CO2 (Ross and Davidson(1982)) C + CO2 → 2 CO (Biggs and Agarwal(1997)) C + H2O → CO + H2 (Hobbs et al. (1992)) C + 2 H2 → CH4 (Hobbs et al. (1992))

Reactor at the department

Reactor – inlet boundaries

outlet wood inflow:

wood T = 150◦C, v = 0.08 cm/s bulk density 0.385 g/cm3

air inflow:

gas species 21% O2, 79% N2

Results - Euler-Euler model

Results – exhaust gas composition

Results - Euler-Euler model

Vol % _{hydrogen carbon monoxide}

carbon dioxide hydrocarbons

Euler-Lagrange / DEM – results

Model assumptions:

proportional particle massloss and shrinking

transient calculation of particle mass-loss and energy no heterogeneous reaction (in progress)

no particle-particle heat transfer no radiation

Euler-Lagrange / DEM – results

Summary and outlook

Euler-Lagrange and Euler-Euler model for wood gasification in fluidized beds

Euler-Euler: reasonable agreement with experimental data Euler-Lagrange: allows detailed investigation of

particle/chemistry/gas phase interactions Refined chemistry

Parallelization of the DEM modell

Quantitative comparison between Euler-Euler, Euler-Lagrange/DEM und experiments

The financial support of the Deutsche Bundesstiftung Umwelt (DBU)

Governing equations – Euler-Euler model

Species mass balance gas phase

∂

∂t(gρgYgs) + ∇ · (gρg~vgYgs) = ∇ · (Dgs∇Ygs) +Rgs

Mass balance solid phase ∂

Governing equations – Euler-Euler model

Momentum balance solid phase

∂ ∂t(mρm~vm)+∇·(mρm~vm~vm) = −εm∇pm+∇·τm− nm X l=1 l6=m ~Iml+~Igm+mρm~g

Governing equations – Euler-Euler model

Energy balance gas phase

gρgcpg  ∂Tg ∂t + ~vg· ∇Tg  = −∇ · ~qg+ Nm X m=1 γgm Tm− Tg
− ∆Hg

Energy balance solid phases

_mρ_mcpm  ∂Tm

∂t + ~vm· ∇Tm